Using wind-tunnel testing, this study validates a design that was globally optimized under uncertainty and used computational fluid dynamics. The computational study determined a design for a 3-D tractor-trailer base (back-end) drag-reduction device that reduces the wind-averaged drag coefficient by 41% at 57 mph (92 km/h). The wind-tunnel testing applies the same method of including some uncertainties, such that the design is relatively insensitive to variation in wind speed and direction, elevation, and installation accuracy. The validation testing shows a 20.1% reduction in wind-averaged drag coefficient, or 1.3% better than a non-optimized commercial design, and is conducted on a 1/24-scale model of the simplified tractor trailer at a trailer-width-based Reynolds number (ReW) of 4.9x105. Test data include both force and pressure measurements on the simplified tractor trailer, as well as pressure measurements on the tunnel wall. Measurements are taken at static side-slip angles to enable wind-averaged calculations. Since the original computations are conducted for a full-scale tractor-trailer at ReW = 4.4x106, this study does not fully validate the computational design due to the wind tunnel limitations and resulting inability to match the ReW; however, the results show qualitative and quantitative improvement over the non-optimized design.